The present invention relates to a sintered, magnetically soft composite, especially for use in solenoid valves, and a process for manufacturing such a composite.
Modern gasoline and diesel engines require increasingly efficient magnetic injection valves in order, for instance, to comply with demands for reduced consumption and reduced pollutants. To that end, known fast-acting magnetic injection valves are manufactured from magnetically soft materials such as FeCr or FeCo alloys or from powdery composites having the greatest possible specific electrical resistance.
However, only a specific electrical resistance of 1 xcexcxcexa9m at most can be achieved in the metallic materials by measures using alloy technology.
Furthermore, it is also already known to use magnetic material made of iron powder and organic binder in valves for diesel injection (Common Rail System). While these materials may have higher specific electrical resistance than the aforementioned magnetically soft alloy materials, they are often fuel and temperature-stable only to a limited degree and are also difficult to work.
When compared to the related art, the sintered, magnetically soft composite according to the present invention, and the process for its manufacture, have the advantage of attaining specific electrical resistances of more than 2 xcexcxcexa9m. Furthermore, the composite according to the present invention is very temperature-resistant and, at the same time, fuel-resistant. Moreover, it is mechanically workable, at least to a limited degree.
The composite according to the present invention also achieves a saturation polarization of approx. 1.6 Tesla, which is comparable to known materials made of iron powder and organic binders.
Thus, it is advantageous that a plurality of known, commercially available powders, especially pure iron powder, phosphatized iron powder, iron-chromium alloy powders, or iron-cobalt alloy powders, can be used as ferromagnetic, powdery starting components.
In the same manner, a plurality of known soft or hard magnetic ferrite powders may likewise be used for the second, ferrimagnetic starting component. Particularly advantageous is the use of oxidic powders such as Fe2O3, of known strontium or barium hard ferrites, or known soft ferrites such as MnZn or NiZn.
To ensure that the ferrite powder preferably used as second starting component is present in the composite at least largely as grain boundary phase after sintering, i.e., that this grain boundary phase surrounds the first starting component at least in certain regions after sintering, it is further advantageous if the average particle size of the powder particles of the ferromagnetic starting component is clearly larger than the average particle size of the powder particles of the ferrite powder.
Furthermore, for increased resistance, it is advantageous that further additives such as silicon, silicon dioxide (SiO2), aluminum or aluminum oxide (Al2O3), can be added to the sintered, magnetically soft composite, thereby allowing an adjustment of the physical characteristics of the manufactured composite within certain limits.
Silicon or silicon dioxide may thus be used advantageously to increase the specific electrical resistance and the permeability of the composite. For instance, aluminum or aluminum oxide is suitable for increasing the specific electrical resistance.
The pressing aid also preferably added to the starting mixture facilitates compression and molding of the starting mixture in a mold cavity. In this context, it is advantageous that this compression aid is completely removed again and vaporized when the binder is removed, so that it does not directly influence the achievable material characteristic values of the material of the sintered, magnetically soft composite obtained.